The present inventors are now developing a Faraday rotator made of a polycrystalline garnet ceramic such as TGG (Tb3Ga5O12). The first challenge in this development is to produce a ceramic having a high light transmittance, and the inventors have successfully achieved, for example, a linear light transmittance of 80.3% at a wavelength of 1500 nm and 78.5% at a wavelength of 600 nm for a 1-mm-thick sample by adding, as a sintering aid, 5 to 1000 mass ppm of Ge calculated as metal to TGG (Patent Literature 1: JP 4878343B).
Meanwhile, a Faraday rotator is required to have not only high translucency, but also a high extinction ratio. In practice, an extinction ratio of 30 dB or more is required. Therefore, the inventors have conducted studies to further improve the extinction ratio of the ceramic described in Patent Literature 1, and as a result, they achieved the present invention.
Another related art will now be described. A polycrystalline garnet ceramic can be applied to an X-ray scintillator, and Patent Literature 2: JP 2012-72331A proposes a ceramic having a composition R3(Al1-xGax)5O12 where R is a rare-earth element, and 0<x<1. Also, this composition formula indicates that the ratio between Al and Ga may be freely changed. In addition, Al and Ga are located at close positions in the periodic system, and have similar ionic radii. Accordingly, the Ga atoms in the ceramic can be replaced with Al atoms, and the crystal distortion, the change in the lattice constant, or the like that results from such replacement can be estimated to be small.